The present invention relates to fluidized beds, and more particularly to a method and apparatus for supporting and starting a fluidized bed.
The use of fluidized beds has been recognized as an attractive means of generating heat. In these arrangements, air is passed through a bed of particulate material, which normally consists of a mixture of inert material and a fossil fuel such as coal, to fluidize the bed and to promote the combustion of the fuel. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed system offers an attractive combination of high heat release, improved heat transfer to surfaces within the bed, and compact boiler size.
When a fluidized bed boiler is activated after a period of idle time, the bed material must be heated above the fuel ignition temperature before operation can begin. However, the mass of material involved makes simultaneous start-up of the entire bed expensive and inefficient, especially with regard to relatively large beds. Although several techniques have been suggested for starting a fluidized bed they each provide less than optimum solutions. For example, since coal is difficult to burn in a fluidized bed until the bed is raised to a relatively high temperature, the use of external burners or the like to heat the bed to the coal burning temperature often takes several hours and is inefficient with respect to fuel use. Also, it has been suggested to preheat the beds with the use of hot flames directed onto the top surface of the bed. However, the high temperatures resulting from the application of the direct flames can injure the particles or the boiler structure. It has also been suggested to provide physical partitions within the beds to divide the bed into a plurality of smaller beds, and start up the entire bed by sequentially starting up each individual bed. However, the use of these partitions considerably adds to the cost of the system.